As of of this blog post date, the current release of Yosemite is OS X 10.10.2. Like other recent OS X releases, Yosemite is a free download in the Mac App Store to anyone running OS X 10.6-10.9 with an Apple ID.
I highly recommend AGAINST upgrading Macs used in a production environment to Yosemite at this time. I’m sure my opinion will change as Apple releases patches in the future, but historically I don’t consider an OS stable until Apple has released the .4 or .5 update (i.e. wait until 10.10.4 at a minimum). Unless you have a SSD or hybrid drive, you’ll likely experience slower performance, plus there are a lot of bugs still being worked out. Also your Mac may become very unstable if you don’t have Yosemite compatible versions of all the applications you use, and this could result in several hundred dollars in upgrade costs.